A common problem to overcome in wireless communications is multipath fading, or simply fading. In a typical wireless communication system, multiple copies or multipaths of a transmitted signal arrive at a receiver from different directions and with different time delays. The multipaths combine at the receiver to yield a resultant received signal that can vary greatly in amplitude and phase, making it difficult to detect and demodulate.
Diversity is a commonly used technique in wireless communication systems to combat fading. There are many forms of diversity, all with the same underlying idea, which is to provide redundant signals at the receiver over independently fading channels. If multiple copies of the same signal are received at the receiver, there is a good likelihood that at least one of the redundant signals will not be degraded by fading, or that the redundant signals can be combined to yield a signal suitable for demodulation.
One type of diversity is known as transmit diversity, wherein multiple copies of the same signal are transmitted using different frequencies, antennas, polarizations, or combinations thereof. When multiple transmitting elements are in relatively close proximity and form an array, the technique is known as coherent transmit beamforming. Patents related to coherent transmit beamforming include U.S. Pat. Nos. 6,088,593; 5,940,742; 5,909,460; 5,848,060; 5,812,947; 6,331,898; 5,619,503; 5,594,941; 5,642,358; and 5,594,941. Coherent transmit beamforming, as described in the above identified patents, assumes that the impulse response of the propagation paths from each antenna element to a given receiver are known to the base station. In “Mobile-Assisted Beamforming,” which is described in the '941 patent, the mobile terminal provides feedback to the base station using a reverse communication channel to assist the base station in the determining channel estimates for the various propagation paths.
Another form of transmit diversity is variously known as simulcast, multicast, and transmit macro-diversity. Transmit macro-diversity employs multiple antennas widely separated from one another to transmit transmit signals to the same receiver. The propagation channel from each antenna to each receiver is a multipath channel having multiple propagation paths of different delay with time-varying amplitude and phase. Non-coherent macro-diversity increases the geographical spread of the interference from a given signal and reduces the reuse of the same channel. It nevertheless increases capacity over the capacity that is available without macro-diversity, as many receivers are located in the border regions between different transmitter service areas, when a uniform area distribution of receivers applies, and thus benefit from macro-diversity. Transmit macro-diversity is described more fully in U.S. Pat. Nos. 6,104,933; 5,940,445; 5,930,248; 5,883,888; 5,845,199; 5,724,666; 5,812,935; 5,327,577; and 3,917,998.
Non-coherent transmit macro-diversity is less effective than coherent beamforming in some important ways. Both coherent beamforming and non-coherent macro-diversity enhance the received signal by summing the transmitted powers of all cooperating transmit antennas or antenna elements. However, only coherent beamforming diminishes the interference transmitted in other directions and focuses the transmitted beam towards the receiver, providing additional antenna gain. With coherent beamforming, the same frequency channel may be re-employed in different directions without interference, increasing communications capacity. However, with traditional coherent beamforming, the receivers for which the same channel is re-employed must be spaced by more than the beamwidth of the transmission beam.
In addition to addressing fading issues, research has shown that wireless systems using multiple transmit antennas and multiple receive antennas can achieve high data rate capacity. The use of multiple antennas to increase data rate capacity is discussed in E. Telatar, Capacity of Multi-antenna Gaussian Channels, AT&T-Bell Labs Internal Tech. Memo, June 1995; G. J. Foschini and M. J. Gans, On Limits of Wireless Communication in a Fading Environment When Using Multiple Antennas, Wireless Personal Communications, vol. 6, (no.3), pp.311–35, Klu-wer Academic Publishers, March 1998; and G. J. Foschini, R. A. Valenzuela, Initial Estimation of Communication Efficiency of Indoor Wire-less Channel, Wireless Networks 3, pp. 141–154, 1997. These articles discuss methods used for increasing the data rate that can be provided to a signal receiver equipped with multiple antennas. Using a channel coefficient matrix that describes the propagation paths from each of N transmit antennas to each of N receive antennas, it is possible to provide N distinct communication channels. Each channel provides a basic data rate, thereby providing N times the basic data rate. The capacity gains can be achieved, however, only when the signals from the N receive antennas are available at a single location.